The reflected power measurement of known microwave generator systems is typically used as an indirect mechanism for determining the level of applied power (given that the energy deposited in the target material cannot be easily obtained directly). In the event that a predetermined level of reflected power is exceeded the generator system can quickly and safely be shutdown, preventing damage to the equipment and preventing faulty devices operating due to a system misreading.
In medical applications the measurement of reflected power can act as a safety mechanism to detect and react to device failures, connection issues and some forms of misuse. The advantage of reflection measurements is that blind treatments can be monitored in real time without requiring the user to inspect the treatment site, which could result in additional power being administered, which could inadvertently cause adverse events. Conversely an entirely adequate medical device could accidentally be misinterpreted as being faulty causing the user to abandon a treatment causing unnecessary risk and distress to a patient and requiring the treatment to be rescheduled.
In known apparatus, forward and reverse power circuits are used to measure the energy delivered to and reflected from load components connected to a microwave generator. The accuracy of that measurement is important as it can be used as a safety monitor or to protect amplifier circuitry in the generator from high levels of reflected power that can damage the hardware.
In the case of a load component that reflects a portion of incident microwave energy, a voltage standing wave is established which varies sinusoidally in amplitude with distance (electrical phase length) from the mismatched component. The ratio of the voltage maximum (antinode) to the adjacent voltage minimum (node) on a transmission line is relative to the proportion of the energy reflected and the energy delivered and is called the voltage standing wave ratio (VSWR).
Typically microwave components are designed and measured using vector (magnitude and phase) network measurement equipment where mismatches and measurement-cable phase effects are calibrated out using sophisticated software based calibration techniques. These components are designed and measured against highly accurate 50Ω reference standards. Typically components will differ slightly from the reference standards presenting a mismatch that will cause some minor degree of VSWR. The issue only becomes significant in cases where the component match is poor. In the case of medical applications the applicator to system match is often worse than the typical industrial component return loss of −20 dB (VSWR 1.22:1) which can result in significant levels of VSWR.
For microwave systems that employ a single operating frequency, the reverse power measurement circuitry will only measure a single point of the VSWR sinusoid. As the VSWR sinusoid varies with distance, changes in length of the path will cause the reflected power measurement to follow the profile of the VSWR sinusoid. This effect could cause the reverse power measurement circuit to measure anywhere from a VSWR maximum to a VSWR minimum for two identical mismatched components differing only in phase length. In a system that relies upon measuring the reverse power to denote performance this represents an ambiguous and unreliable measurement.
In known generator systems, microwave power is often measured using detector diodes which provide a voltage related power measurement. These systems are constructed from combinations of components designed and measured against accurate 50Ω impedance reference standards using vector (magnitude and phase) test equipment, however the finished systems are expected to make critical measurements without reference to any form of in situ calibration. Instead, operating characteristics of systems are determined from measured reflected signals (often measured at a single location and frequency) based upon properties of individual components pre-determined against the reference standards. The omission of calibration emphasises the effects of variation and cable phase length upon the power measurement. It is also common practice to generate microwave signals using continuous wave (CW) microwave generators. Such CW generators are often limited to generating microwave energy at a single fixed frequency point, for example 2.45 GHz, due to the use of magnetron based technology that can only provide energy at fixed frequencies. Variation of device physical parameters in conjunction with CW operation and lack of calibration can result in considerable variation in measurements.
Another factor that is overlooked in known systems is the effect of cascaded mismatches. For example, components designed for a 50Ω termination may be connected to other mismatched components and the electrical phase length of the interconnects such as cables or phase length of the applicators may be either ignored or unknown causing mismatch uncertainty. That is an often overlooked source of error in microwave measurements. The design and calibration of equipment, particularly medical equipment should take account of measurement uncertainties to demonstrate competence.
It should be understood that, disregarding the aforementioned effects of VSWR and standing waves on reflected power measurements, the overall power delivered to a load should remain relatively constant irrespective of minor variations in cable length provided the load is of a constant value.
It is known to measure reverse power using a directional coupler connected to a detector diode. VSWR is computed by establishing the ratio of forward to reverse power measured. In most systems reverse power standing wave is measured at a single frequency point. The effect of the standing wave is not evident at this single frequency point measurement until parameters such as match or phase length are varied (as often happens with component manufacturing tolerances).
A limitation in using couplers or impedance dependant measuring components in power detector circuits is that these devices are often suited to measuring against matched terminations. In the case of directional couplers the coupling factor and the directivity will be affected by the impedance presented at the ports of the device. The performance of a detector circuit that uses a coupler or any other arrangement of impedance sensitive components (e.g. coupler/isolator) will be affected by the varying impedance presented to the ports.
This characteristic is acceptable in industrial applications where the device under test (DUT) typically possesses a match of −14 dB or better (VSWR 2:1). In medical applications the applicator or antenna match can differ considerably from 50Ω and may range from −20 dB (VSWR 1.22:1) to −6 dB (VSWR 3.01:1), or worse depending upon the application. The effective impedance of the applicator or antenna can also change as the properties of tissue change during a treatment which requires a measurement system that can accommodate a wide range of impedance variation. This is particularly critical in reflected power measurements where a medical system has been configured to measure reflected power using standard 50Ω reference components. When connected to a new impedance the setup will continue to refer all measurements to the 50Ω reference standard resulting in uncertainty and unreliability of reflected power measurements where the impedance differs from 50Ω. This impedance related measurement limitation may also be subject to the effects of phase which can be reduced using a swept source as described herein or by any other method that takes an average measurement over variations in phase by either mechanically or electrically sweeping the phase. The effect of impedance sensitive measurement variation on its own adds a further independent source of error into power measurements.
Systems that utilise reflected power measurements and determination of VSWRs in monitoring and controlling the application of microwave power for medical applications are described in US20090076492, U.S. Pat. No. 7,070,595, U.S. Ser. No. 11/479,259, and US 20080319434.
US 20090076492 and U.S. Pat. No. 7,070,595 describe adjusting a system parameter, such as phase length, or moving an output frequency, so as to operate at a position of lowest measured VSWR, which is perceived as the optimal operating arrangement. However that approach is flawed in that the overall system performance is not being improved as the antenna impedance remains unchanged and the power delivered remains the same. The only change is that the reference point of the VSWR measurement is moved to a null point at which the reflected signal is partially cancelled against the transmitted signal. Thus, such systems provide for unreliable measurement of reflected power or VSWR, which cannot be relied upon for safety critical purposes.